Video gets transcoded if file contains ASS subtitles

[LinAGKar]

Describe the bug

If a video file contains ASS subtitles, the video is always transcoded, although the same does not happen when it contains SRT subtitles.

To Reproduce

1. Have an mkv file with AVC video, AAC audio, and ASS subtitles
2. Play the file on Chromecast
3. Video will be transcoded, even if the Chromecast could play the video.
4. Try exactly the same video, but with SRT subtitles (I just ran the file through ffmpeg with -c:a copy -c:v copy -c:s srt -map 0)
5. Video is played with DirectStream. Audio still got transcoded (and downmixed from 5.1 to 2.0), but video got copied.

Expected behavior

Video should not be unnecessarily transcoded

System (please complete the following information):

• OS: Docker
• Jellyfin Version: 10.8.0-beta1
• Cast client: Gen 2

[3flex]

I don't think there's anything that can be done on the Chromecast side.

Google Cast Framework only supports these subtitle formats:

• TTML - Timed Text Markup Language
• WebVTT - Web Video Text Tracks
• CEA-608/708

In jellyfin-chromecast source it says that SRT can be converted to VTT by Jellyfin before delivering to the Cast device which is why video transcoding is not required for SRT - because Jellyfin actually delivers VTT to the Cast device which handles that format natively. Some subtitle formats, such as ASS, possibly cannot be converted without embedding in the file.

jellyfin-chromecast/src/helpers.ts

Lines 412 to 413 in 50c6854

	/* The CAF v3 player only supports vtt currently,
	* SRT subs can be "transcoded" to vtt by jellyfin.

Also check transcoding settings. The option "Allow subtitle extraction on the fly" should be enabled.

I tried digging in jellyfin/jellyfin source & docs but couldn't find anything clearly stating which subtitle formats can be converted to VTT without video transcoding. If this isn't working correctly a bug should be opened on jellyfin/jellyfin.